What is a common effect of over-watering plants?

Over-watering plants is a common issue that can lead to root rot, making this the correct answer. When plants receive too much water, the soil becomes saturated, which prevents not only proper drainage but also the flow of oxygen to the roots. Roots need oxygen to function effectively, and without it, they can begin to decay. Root rot is characterized by the roots turning brown or black, becoming mushy, and ultimately leading to plant death if not addressed quickly.

In contrast, while promoting root growth and increasing soil aeration are beneficial aspects of proper watering, over-watering does not contribute to these outcomes. Instead, it creates a waterlogged environment that suffocates roots. Additionally, while over-watering can cause other problems such as certain nutrient deficiencies, primarily due to the plant's inability to take up nutrients efficiently in an oxygen-depleted environment, root rot is the most direct and severe consequence of excessive moisture in the soil.